Molluscs

Read about the sandbowl snail on the Northern Devon sand dune invertebrates group page.

Habitat 

Freshwater Pearl Mussels live in fast-flowing, nutrient-poor streams and rivers with clean cobbles and gravels. They’re one of the longest living invertebrates, capable of living to over 130 years. They can form mussel beds which provide habitat for other wildlife and, as the mussels are filter feeders, clean the water.  

To reproduce, females release larvae into the water, which attach to the gills of trout or salmon for aound 9 months. The larvae then drop off and settle in clean gravel beds where they grow into full-sized mussels, reaching maturity at around 12 years. They use their gills to catch tiny bits of plants and animals floating in the water.   

Freshwater Pearl Mussels need rivers with high water quality (sediments clog their gills), stable gravel beds and high oxygen levels. Significant changes in river flow can also harm them. Low flows can dry out gravels and leave the mussels stranded, while floods can wash them into unsuitable areas.  

Population 

Freshwater Pearl Mussels were once widespread in our rivers. Their pearls have been traded since Roman times, and in the 16th century river bailiffs were employed to ensure all valuable pearls were kept for the king.  However, this historic exploitation for pearls along with a decline in the quality of river habitat has led to huge declines: 90% across their UK range by the end of the 20^th century. They’re now globally threatened.   

In south-west England the only remaining populations are on the lower sections of the Taw and Torridge rivers, but in very low and decreasing numbers. These populations have not bred successfully in the wild since the 1960s and so the remaining individual mussels are now over 80 years old. The Torridge population may be the oldest population in UK, and likely dates back between 20,000 and 30,000 years.  

Habitat in the Taw and Torridge is now very degraded. Most cobbles and gravels have been eroded away or are smothered by fine silts. Juvenile mussels were reintroduced in the lower Torridge in 2017-18 through a Devon Wildlife Trust project. However, the success of this cannot be confirmed until at least 2027 when the mussels will be big enough to see.  The project developed a ten year strategy but no funding has been available since 2018.  The population will be extinct if no action is taken. 

For a detailed overview please read the Freshwater Pearl Mussel paper written for the LNRS. See Find out more below. 

This group of rare snails lives in woodlands in areas with heavy rainfall and mild climates. A large percentage of the world’s population is therefore found in old oak woodlands on the western Atlantic fringes of Europe.  

The snails are largely found in cool, damp areas within woodlands on bark, dead wood or in leaf litter. In Devon they’re largely found in the upland rainforest oak woods on and around Dartmoor and Exmoor.  

Pressures include impacts to this damp habitat due to woodland management (especially loss of deadwood) and climate change. The following are Devon Species of Conservation Concern.

The Spiny Cockle is found in the sandy substrates of Devon’s shallow coastal waters where it can burrow into the sand to avoid predation and harsh environmental conditions.  Spiny Cockles are filter feeders. They filter plankton and small particles from the water, improving water quality and recycling nutrients.   

They’re easily recognisable by their robust, heart-shaped shell with prominent radial ribs and spines. These spines, which give the species its common name, serve as a protective barrier against predators. The shell’s exterior ranges in colour from pale yellow to brown, often with darker bands or streaks. The interior of the shell is smooth and typically white or light-coloured.  

Devon has a higher recorded population of Spiny Cockles compared to other regions in the UK, which is particularly evident in areas like the Salcombe estuary. Despite its relatively high numbers in Devon, the Spiny Cockle is considered a nationally rare species.  

The Celtic Sea Slug is a small, air-breathing mollusc found on north Devon’s rocky shores from April to November, especially in areas like Woolacombe, Saunton and Croyde. It shelters in crevices and among barnacles and mussels, emerging at low tide to graze on microscopic algae, diatomsA major group of unicellular algae found in oceans, lakes, and rivers. and detritus. It’s often seen near whelk eggs, feeding on algae growing around the egg cases. 

Dark green-black and well-camouflaged, the slug’s body is oval and fleshy, up to 13 mm long, and covered in coarse, evenly spaced rounded bumps that help it blend into its surroundings. 

Unlike typical marine sea slugs that breathe through gills, the Celtic Sea Slug breathes air through a lung-like cavity. This is because it evolved from a land-dwelling or amphibious slug, rather than a gilled marine ancestor. This adaptation makes the slug uniquely suited to life in the intertidal zone, allowing it to survive for long periods out of water during low tide and helping it thrive in north Devon’s rugged coastal habitats. 

Though small and often overlooked, the Celtic Sea Slug plays an important ecological role and is a sign of a healthy, balanced shoreline ecosystem. 

Native Oysters start life as males and can change sex based on their environment. They reach maturity at between eight and ten months old and can live up to six years, with some living as long as 15 years. They feed by filtering water to extract nutrients; a single adult oyster can filter over 140 litres of water a day. 

Native Oysters live in estuaries and shallow coastal waters with mud, rocks, sandy mud and gravel and can be found from the low tide line down to about 80 m deep.

They grow together in what are known as oyster beds or reefs. The oysters start life as larvae, which drift in the water. After a few weeks they find a hard surface (rocks, structures such as piers or the shells of older oysters), which they permanently cement themselves onto. As more and more oysters cement themselves to each other they create layers of living and dead oyster shells that form the oyster beds.   

Native Oyster beds have declined due to pollution and historic overharvesting. A current issue is the threat from the invasive non-native Pacific Oyster (Magallana gigas). These were introduced as a farmed oyster and can create dense reefs. They compete with Native Oysters for space and food and have caused a further decline in Native Oyster populations. Controlling the spread of Pacific Oysters is challenging and resource intensive. Efforts to manage their populations include culling and monitoring, but these measures require significant effort and coordination.

Native oysters are also threatened by diseases such as Bonamia ostreae, a parasite that infects their blood cells.